The present invention relates generally to a display to be used in a portable device and, more specifically, to a display that uses one or more diffractive elements for extending the exit pupil of the display for viewing.
While it is a common practice to use a low-resolution liquid-crystal display (LCD) panel to display network information and text messages in a mobile device, it is preferred to use a high-resolution display to browse rich information content of text and images. A microdisplay-based system can provide full color pixels at 50-100 lines per mm. Such high-resolution is generally suitable for a virtual display. A virtual display typically consists of a microdisplay to provide an image and an optical arrangement for manipulating light emerging from the image in such a way that it is perceived as large as a direct view display panel. A virtual display can be monocular or binocular.
The size of the beam of light emerging from imaging optics toward the eye is called exit pupil. In a Near-Eye Display (NED), the exit pupil is typically of the order of 10 mm in diameter. Further enlarging the exit pupil makes using the virtual display significantly easier, because the device can be put at a distance from the eye. Thus, such a display no longer qualifies as a NED, for obvious reasons. Head-Up Displays are an example of the virtual display with a sufficiently large exit pupil.
WO 99/52002 discloses a method of enlarging the exit pupil of a virtual display. The disclosed method uses three successive holographic optical elements (HOEs) to enlarge the exit pupil. In particular, the HOEs are diffractive grating elements arranged on a planar, light transmissive substrate 6, as shown in FIG. 1. As shown, light from an image source 2 is incident upon the first HOE, or H1, which is disposed on one side of the substrate 6. Light from H1, coupled into the substrate 6, is directed toward the second HOE, or H2, where the distribution of light is expanded in one direction. H2 also redirects the expanded light distribution to the third HOE, or H3, where the light distribution is further expanded in another direction. The holographic elements can be on any side of the substrate 6. H3 also redirects the expanded light distribution outward from the substrate surface on which H3 is disposed. The optical system, as shown in FIG. 1, operates as a beam-expanding device, which maintains the general direction of the light beam. Such a device is also referred to as an exit pupil extender (EPE).
The EPE, such as that shown in FIG. 1, results in color non-uniformity, thereby degrading the quality of the reproduced virtual image. The color non-uniformity is mainly due to the fact that light beams of different colors travel different paths in the substrate 6, as shown in FIG. 2. For illustration purposes, only two colors, represented by xcex1 and xcex2, are used to show the source of color non-uniformity in the prior art EPE, with xcex1 less than xcex2.
In FIG. 2, only two HOEs are used, but the source of color non-uniformity is the same when three or more HOEs are used. The first HOE, or H1, typically has a diffractive structure consisting of parallel diffractive fringes for coupling incident light into the substrate 6 and directing the light distribution within the substrate 6 toward the second HOE, or H2. The substrate 6 acts as a light guide to trap the light beams between its two surfaces mainly by means of total internal reflection (TIR). As shown in FIG. 2, the diffractive elements H1 and H2 are both disposed on the lower surface of the substrate 6. In such an optical device, TIR is complete only at the upper surface, because part of the light is diffracted out from the lower surface of the substrate toward the viewer""s eye.
It is known that the diffraction angle inside the substrate 6 is governed by:
sin(xcex8i)xe2x88x92n sin(xcex8m)=m xcex/dxe2x80x83xe2x80x83(1)
where d is the grating period of the diffractive element (here H1)
xcex is the wavelength
n is the refractive index of the substrate
m is the diffraction order
xcex8i is the angle of incident, and
xcex8m is the angle of diffraction in mth order.
As can be seen from Equation 1, the diffraction angle xcex8m increases with wavelength xcex. Thus, the diffraction angle xcex8m1 is smaller than the diffraction angle xcex8m2. As a result, the interval L between two successive TIRs also varies with wavelength. The interval L1 for xcex1 is smaller than the interval L2 for xcex2. Thus, the distribution of outgoing light in the xcex7 direction is not uniform for all wavelengths (see FIG. 6), although the grating structure can be designed so that the output is homogeneous for one wavelength (blue, for example; see FIG. 6). As can be seen in FIG. 2, the shorter wavelength xcex1 experiences more xe2x80x9chitsxe2x80x9d than and xcex2 on the diffractive elements H2. Consequently, more light of the shorter wavelength xcex1 xe2x80x9cleaksxe2x80x9d out of the diffractive element H2 in the area near H1. In a display where three primary colors (red, green, blue) are used, an EPE of FIG. 2 will cause an uneven color distribution of the light exiting the diffractive grating structure of H2. Thus, the color may appear bluish on the near end and reddish on the far end, relative to H1. As the distance along the xcex7 direction increases, the uneven color distribution becomes more noticeable.
It should be noted that light can xe2x80x9cleakxe2x80x9d out of the substrate 6 from the lower surface where H2 is located or from the upper surface. The distribution of outgoing light from the upper surface is similar to that from the lower surface.
It is advantageous and desirable to provide a method and system for improving the color uniformity in light distribution in an exit pupil extender.
It is a primary objective of the present invention to reduce or eliminate the difference in the interval between successive total internal reflections for different wavelengths. This objective can be achieved by using a substrate with a plurality of layers so that the total internal reflection for one color occurs at a different layer surface.
Thus, according to the first aspect of the present invention, there is provided an optical device (10), which comprises:
a substantially planar light-guiding member (60) having a first surface (32) and an opposing second surface (52), and
a light coupling structure (H1) positioned relative to the light-guiding member for coupling light waves (70) into the light-guiding member, the light-guiding member guiding the light waves (72, 74) within the light-guiding member based substantially on successive internal reflections as the light waves travel between the first and second surfaces, the light waves comprising at least first light waves of a first color (xcex1) and second light waves of a second different color (xcex2), wherein the first light waves are internally reflected at a first reflection angle (xcex8m1) and the second light waves are internally reflected at a second reflection angle (xcex8m2) greater than the first reflection angle in reference to a surface normal (N) of the second surface. The optical device is characterized by at least one substantially planar interface (40) provided between the first surface and the second surface substantially parallel to the first surface to reflect the second light waves toward the second surface as the second light waves travel from the second surface toward the first surface, while allowing the first light waves traveling from the second surface toward the first surface to be transmitted through the planar interface.
Preferably, when the successive internal reflections by the first light waves at the second surface occur at a plurality of first reflection points separated by a first reflection interval (L1), and the successive internal reflections by the second light waves at the second surface occur at a plurality of second reflection points separated by a second reflection interval (L2), the optical device is further characterized in that the planar interface is positioned between the first and second surfaces such that the first reflection interval is substantially equal to the second reflection interval.
Preferably, the optical device further comprises a further light coupling structure (H2) positioned relative to the light-guiding member to cause the light waves encountering the second surface to be partially transmitted through the second surface and partially reflected toward the first surface. The further light coupling structure (H2) is also capable of causing the light waves encountering the second surface to be partially transmitted through the planar interface (40) and then the first surface (32), and partially reflected from the second surface toward the first surface while maintaining the reflection angles.
Preferably, the light coupling structure (H1) and the further light coupling structure (H2) are holographic diffractive elements imparted on the light-guiding member.
Preferably, when the light waves further comprise third light waves of a third color (xcex3) different from the first and second colors, and the third light waves are internally reflected at a third reflection angle (xcex8m3) smaller than the first and the second reflection angles, the optical device is further characterized by
a further planar interface (42) provided between the first surface (32) and the planar interface (40) so as to reflect the first light waves toward the second surface as the first light waves travel from the second surface toward the first surface, while allowing the third light waves traveling from the second surface toward the first surface to be transmitted through the further planar interface.
Preferably, when the light-guiding member is made of an optical material having a first refractive index, the planar interface (40) is a layer made of an optical material having a second refractive index smaller than the first refractive index so that the reflections by the second light waves at the planar interface are total internal reflections.
Preferably, the further planar interface (42) is a layer made of an optical material having a second refractive index smaller than the first refractive index so that the reflections by the first light waves at the further planar interface are total internal reflections.
According to the second aspect of the present invention, there is provided a method of improving color uniformity in an optical device (10), wherein the optical device comprises:
a substantially planar light-guiding member (60) having a first surface (32) and an opposing second surface (52), and
a light coupling structure (H1) positioned relative to the light-guiding member for coupling light waves (70) into the light-guiding member, the light-guiding member guiding the light waves (72, 74) within the light-guiding member based substantially on successive internal reflections as the light waves travel between the first and second surfaces, the light waves comprising at least first light waves of a first color (xcex1) and second light waves of a second different color (xcex2), wherein the first light waves are internally reflected at a first reflection angle (xcex8m1) and the second light waves are internally reflected at a second reflection angle (xcex8m2) greater than the first reflection angle in reference to a surface normal (N) of the second surface. The method is characterized by
providing at least one substantially planar interface (40) between the first surface and the second surface substantially parallel to the first surface so as to reflect the second light waves toward the second surface as the second light waves travel from the second surface toward the first surface, while allowing the first light waves traveling from the second surface toward the first surface to be transmitted through the planar interface.
Preferably, when the successive internal reflections by the first light waves at the second surface occur at a plurality of first reflection points separated by a first reflection interval (L1), and the successive internal reflections by the second light waves at the second surface occur at a plurality of second reflection points separated by a second reflection interval (L2), the method is further characterized in that
the planar interface is provided at a position between the first and second surfaces such that the first reflection interval is substantially equal to the second reflection interval.
According to the third aspect of the present invention, there is provided a substantially planar waveguide (60) having a first surface (32) and an opposing second surface (52), to be used with a first light coupling structure (H1) and a second light coupling structure (2), wherein
the first light coupling structure is positioned relative to the planar waveguide for coupling light waves (70) into the planar waveguide, the planar waveguide guiding the light waves (72, 74) based substantially on successive internal reflections as the light waves travel between the first and second surfaces, the light waves comprising at least first light waves of a first color (xcex1) and second light waves of a second different color (xcex2), wherein the first light waves are internally reflected at a first reflection angle (xcex8m1) and the second light waves are internally reflected at a second reflection angle (xcex8m2) greater than the first reflection angle in reference to a surface normal (N) of the second surface, and
the second light coupling structure (H2) is positioned relative to the planar waveguide to cause the light waves encountering the second surface to be partially transmitted through the second surface and partially reflected toward the first surface. The planar waveguide is characterized by
at least one substantially planar interface (40) provided between the first surface and the second surface substantially parallel to the first surface to reflect the second light waves toward the second surface as the second light waves travel from the second surface toward the first surface, while allowing the first light waves traveling from the second surface toward the first surface to be transmitted through the planar interface.
Preferably, when the successive internal reflections by the first light waves at the second surface occur at a plurality of first reflection points separated by a first reflection interval (L1), and the successive internal reflections by the second light waves at the second surface occur at a plurality of second reflection interval (L2), the planar waveguide is further characterized in that
the planar interface is positioned between the first and second surfaces such that the first reflection interval is substantially equal to the second reflection interval.
Preferably, when the light waves further comprise third light waves of a third color (xcex3) different from the first and second colors, and the third light waves are internally reflected at a third reflection angle (xcex8m3) smaller than the first and the second reflection angles. The planar waveguide is further characterized by
a further planar interface (42) provided between the first surface (32) and the planar interface (40) so as to reflect the first light waves toward the second surface as the first light waves travel from the second surface toward the first surface, while allowing the third light waves traveling from the second surface toward the first surface to be transmitted through the further planar interface.